Yeast CO2 Production and Bread Leavening Chemistry

Leavened bread appears in Egyptian records from around 3000 BCE, almost certainly discovered when wild yeast contaminated stored grain paste. Controlled baking cultures developed in Europe through monastic and guild traditions, though the biochemistry behind the rise wasn't articulated until Pasteur identified yeast as a living organism responsible for fermentation in 1857.

Bread rises because Saccharomyces cerevisiae — and in sourdough cultures, a mixed population of wild yeasts and lactic acid bacteria — consumes fermentable sugars and excretes carbon dioxide and ethanol as metabolic waste. That CO2 gets trapped in the protein network of gluten: the elastic-plastic matrix formed when glutenin and gliadin proteins hydrate and align under mechanical work. Without developed gluten, gas escapes freely and the loaf stays flat. Without live yeast, there's no gas to trap. The biochemical chain starts the moment yeast contacts water and available sugars. Flour carries its own amylase enzymes that break damaged starch into maltose; yeast secretes maltase to cleave that into glucose, which enters glycolysis. At temperatures between 25°C and 35°C, S. cerevisiae works aerobically at first, burning glucose to CO2 and water, then shifts to anaerobic fermentation once oxygen is depleted, producing CO2 and ethanol. The optimal enzymatic rate sits around 27°C — McGee notes that below 10°C fermentation slows dramatically but doesn't stop, which is the mechanical basis for cold retarding dough overnight. Gluten development is the other half of the equation. A gas cell surrounded by weak, poorly hydrated gluten will simply pop under pressure. A well-developed gluten network stretches around expanding CO2 bubbles, holding structure through proof and into the oven. Myhrvold and Young in Modernist Cuisine describe this network as viscoelastic — it needs to be elastic enough to stretch without tearing, viscous enough to hold shape. That balance is controlled by hydration, flour protein content, mixing time, and fermentation duration. In the oven, yeast activity spikes one final time between 30°C and 50°C — the oven spring — before cells die above 60°C. CO2 from residual fermentation and thermal expansion of trapped gases drive final volume increase. Starch gelatinizes, gluten sets, and the crumb structure is fixed. Understanding where each of these transitions happens lets you engineer the loaf deliberately rather than guessing.

The flavour of leavened bread is a by-product of fermentation biochemistry and Maillard chemistry. Ethanol produced alongside CO2 reacts with organic acids during baking to form esters — fruity, buttery aromatic compounds. Longer, cooler fermentation allows yeast to also produce diacetyl and acetaldehyde, which contribute buttery and green-apple notes at threshold concentrations. In sourdough, lactic acid bacteria produce lactic acid (clean, yoghurt-like acidity) and acetic acid (sharp, vinegary bite) in a ratio determined by hydration and fermentation temperature — drier, cooler doughs favour acetic acid production. Maillard reactions in the crust between reducing sugars (released by amylase activity throughout fermentation) and amino acids produce hundreds of volatile aromatic compounds — melanoidins, pyrazines, furans — that constitute the signature smell and taste of baked bread crust.